The present invention relates to a new process for producing carbonic acid diaryl esters in which the carbonic acid diaryl esters are produced by reacting monophenols and phosgene in an inert solvent in the presence of alkali and a nitrogen catalyst in the interfacial process.
A two stage interfacial polycondensation process for the production of carbonic acid diaryl ester is disclosed. The invention concerns an improvement to the process that entails forming in a first stage a reaction mixture that contains an inert solvent, at least one monophenol, phosgene, and aqueous alkali hydroxide solution, passing said reaction mixture to a second stage through at least one constriction at a velocity of 3 to 15 m/s under conditions designed to expose the mixture to a pressure drop of 0.1 to 2.5 bar, and in said second stage passing said mixture through at least one dispersing element at a velocity of 2 to 10 m/s under conditions designed to expose the mixture to a pressure drop of 0.1 to 0.5 bar. The resulting product is characterized by its purity and suitability for the manufacture of polycarbonates in the melt transesterification process.
The production of carbonic acid diaryl esters by the interfacial process is basically known from the literature, cf. Chemistry and Physics of Polycarbonates, Polymer Reviews, H. Schnell, Vol.9, John Wiley and Sons, Inc. (1964), p. 50/51). It is known to a person skilled in the art and described in the above-mentioned literature and, for example, in U.S. Pat. No. 4,016,190, that thorough mixing of the two phases is required for reaction progress. However, satisfactory results with respect to yield and purity, in particular the chloroformic acid aryl ester content of the carbonic acid diaryl ester obtained are not always achieved with the known processes. Starting from the state of the art, in particular U.S. Pat. No. 4,016,190, the object was to provide a process which allows better yields and purities of carbonic acid diaryl ester. It is particularly important to avoid entraining the chloroformic acid aryl ester into the end product as this compound interferes during the melt transesterification process to form the polycarbonate.
It has now surprisingly been found that, during the continuous production of carbonic acid diaryl esters by reaction of monophenols and phosgene in an inert solvent in the presence of alkali in the interfacial polycondensation process (herein interfacial process) it is necessary to maintain specific mixing conditions in order to achieve an optimum reaction in short reaction times with high yields and satisfactory purities of the carbonic acid diaryl ester at low temperatures.
It has also been found that a mixing range which is optimum in each case exists as a function of the progress of the reaction stage.
The alkali used may be a lye solution (that is an aqueous solution of any of Na, K, Li, or Ca-hydroxide), preferably sodium hydroxide solution, and is preferably used as a 20 to 55 wt. %, particularly preferably 30 to 50 wt. % solution in the process according to the invention.
Phosgene may be used in liquid or gaseous form or dissolved in the inert solvent.
Suitable monophenols for use in the reaction include phenols of formula (I) 
wherein
R is hydrogen, tert.-butyl, halogens or a branched or unbranched C8 and/or C9 alkyl radical,
therefore phenol itself, alkylphenols such as cresols, p-tert.-butylphenol, p-cumylphenol, p-n-octylphenol, p-iso-octylphenol, p-n-nonylphenol and p-iso-nonylphenol, halogen phenols such as p-chlorophenol, 2,4-dichlorophenol, p-bromophenol and 2,4,6-tribromophenol may be used. Phenol is preferred.
Inert organic solvents used in the process include, for example, dichloromethane, toluene, the various dichloroethanes and chloropropane compounds, chlorobenzene and chlorotoluene, dichloromethane preferably being used.
Reaction control is preferably carried out continuously and preferably in a plug flow without significant back-mixing. This may be effected, for example, in tubular reactors. Reaction control is split into two successive stages.
In the first stage, the aqueous phase is mixed with the organic phase in a diaphragm, a nozzle or a dynamic mixer. It is preferable to use a nozzle, for example a combined confuser/diffuser provided with apertures or an abruptly tapered tube.
In said first stage the aqueous phase flows through the apertures at velocities between 2 and 15 m/s, preferably between 6 and 12 m/s. The pressure drop varies between 0.1 and 2.5 bar, preferably between 0.5 and 1.5 bar. In the narrowest cross-section the mixture has a velocity of between 2 and 15 m/s and a pressure drop of between 0.1 and 2 bar.
The reaction takes place in a reactor that is equipped with cooling means, preferably a liquid distributor, to cool the reaction mixture to a temperature lower than 50xc2x0 C., preferably lower than 40xc2x0 C., downstream of the mixing elements.
The mixing conditions in the first and second stage of reaction ensure high phosgene yields, high-purity carbonic acid diaryl ester, prevent high concentrations of the chloroformic acid aryl ester intermediate product in the end product and require but short reaction times. With lower mixing performance, the reaction is incomplete, i.e. the monophenol has not reacted fully with the phosgene so, on the one hand, monophenol passes into the waste water and, on the other hand, the intermediate product of the chloroformic acid aryl ester is entrained into the end product. A high saponification cleavage and back-cleavage of the phosgene and carbonic acid diaryl ester occurs with excessively high mixing performances, so, on the one hand, monophenol passes into the waste water and, on the other hand, a much higher excess of phosgene is required.
During the reaction in the first stage, the reaction components are initiated by combining the educts phosgene, inert solvent, which preferably serves only as solvent for the phosgene, and phenol, which has preferably already been dissolved beforehand in the caustic solution. The residence time in the continuous process of the first stage is in the range of 2 seconds to 300 seconds, preferably in the range of 4 seconds to 200 seconds. The pH of the first stage is preferably adjusted by the ratio of caustic/phenol/phosgene so the pH lies in the range of 11.0 to 12.0, preferably 11.2 to 11.8.
In the second stage of the continuous process, the reaction is completed to form the carbonic acid diaryl ester. Thorough mixing of the two phases (aqueous and organic phase) in the second stage is carried out by preferably static or dynamic dispersing elements located at regular distances. Preferred static elements include diaphragms, nozzles or constricted tubular cross-sections. Fittings (static mixers) may also be inserted therein to distribute and mix the flow. The pressure drop per dispersing element is preferably between 0.1 and 0.5 bar. The velocity in the narrowest cross-section of the element is typically between 2 and 10 m/s, preferably between 3 and 9 m/s, quite particularly preferably between 4 and 7 m/s. The mixing time (residence time in the dispersing element) is less than 0.5 s, preferably between 0.01 and 0.1 s. The residence time between two successive dispersing elements is between 3 and 12 s, preferably between 5 and 10 s.
The residence times of the second stage in the process according to the invention are between 1 minute and 30 minutes, preferably between 2 minutes and 20 minutes, quite particularly preferably between 3 minutes and 15 minutes.
Suitable dynamic mixers include, for example, pumps or generally rotor/stator systems.
In a preferred embodiment, a catalyst is supplied at the beginning of the second stage. The reaction of the process is preferably cooled directly after or during addition of the catalyst. The reaction temperature is kept  less than 50xc2x0 C., preferably  less than 40xc2x0 C., quite particularly preferably  less than 35xc2x0 C. by cooling. It may be advantageous to add the catalyst at a plurality of, preferably two, points of the second stage of the reaction in the continuous process.
Intermediate buffering may also be carried out between the first and second stage while maintaining the mixed phases.
The catalysts to be used according to the invention are tertiary amines, N-alkyl piperidines or onium salts. Onium salts in the present application are interpreted as compounds such as NR4X, wherein R is an alkyl and/or aryl radical and/or an H and X is an anion. Tributylamine, triethylamine and N-ethylpiperidine are preferably used. N-ethylpiperidine is quite particularly preferred. The concentrations of the catalysts are 0.0001 mol % to 0.1 mol %, preferably 0.01 mol % to 0.075 mol %, based on the phenol used.
In the process according to the invention, the content of chloroformic acid aryl ester in the resultant carbonic acid diaryl ester is  less than 2 ppm, preferably  less than 0.5 ppm.
The pH is preferably regulated in the second stage of the process by measuring the pH (which is preferably measured online in the continuous process) and corresponding adjustment of the pH by addition of the caustic solution. The quantity of caustic solution supplied is adjusted in such a way that the pH lies in the range of 7.5 to 10.5, preferably 8 to 10 after the second stage of the process.
In the process according to the invention, phosgene is supplied in a ratio of 1.01 to 1.14 mol %, preferably 1.05 to 1.10 mol %, based on the phenol. The solvent is added in such a way that the carbonic acid diphenyl ester exists in a 5 to 60% solution, preferably 20 to 45% solution after the reaction.
After the reaction, the organic phase containing the carbonic acid diaryl ester is usually washed with an aqueous liquid and separated as far as possible from the aqueous phase after each washing procedure. Washing is preferably carried out with de-ionized water. The carbonic acid diaryl ester solution is usually cloudy after washing and separation of the washing liquid. Aqueous liquids, for example dilute mineral acids such as HCl or H3PO4 are used to separate the catalyst and water which is completely de-ionized is used for further purification. The concentration of HCl or H3PO4 in the washing liquid may be, for example, 0.5 to 1.0 wt. %. The organic phase is preferably washed twice by way of example.
Basically known separating vessels, phase separators, centrifuges or coalescers or also combinations of these devices may be used as phase-separating devices for separating the washing liquid from the organic phase.
The solvent is evaporated to obtain the high-purity carbonic acid diester. Evaporation may be carried out in a plurality of evaporator stages. For example, it is carried out through one or more successive distillation columns in which the solvent is separated from the carbonic acid diaryl ester.
This purification stage or stages may be conducted, for example, continuously in such a way that the bottom temperature during distillation is  greater than 150xc2x0 C. to 310xc2x0 C., preferably  greater than 160 to 230xc2x0 C. The pressures required to carry out these distillation processes are between 1 and 1000 mbar, preferably between 5 and 100 mbar.
The resultant carbonic acid diesters are characterized/distinguished by very high purity with GC purities (cool on column method)  greater than 99.99%, preferably 99.9925%, quite particularly preferably  greater than 99.995% and extremely good transesterification behaviour, so a polycarbonate of excellent quality may be produced.
The production of aromatic oligo/polycarbonates by the melt transesterification process is known from the literature and has already been described, for example, in the Encyclopedia of Polymer Science, Vol. 10 (1969), Chemistry and Physics of Polycarbonates, Polymer Reviews, H. Schnell, Vol 9, John Wiley and Sons, Inc. (1964) or U.S. Pat. No. 5,340,905.